tcl/target/stm32f4: ramp up JTAG speed, HSI is 16MHz there

Since all F4 parts have an internal HSI providing 16MHz, it's safe to
use 2MHz JTAG frequency by default.

Change-Id: I2702d5a1d642d4acd4af2db54c028949132c6900
Signed-off-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2383
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>